Three dimensional images as used in most 3D display technologies, e.g. EP 0 602 934, EP 0 656 555, EP 0 708 351, EP 0 726 482, EP 0 829 743 and EP 0 860 728 are formed by two 2D images. These displays direct separate images to either eye so that one eye sees one image and the other eye sees the other image. The Human Visual System (HVS) internally fuses the two images into a 3D image so that a person may sense depth from the displayed images.
In order to make stereoscopic images which may be viewed comfortably, the two images must be well aligned. This can be accomplished either by very accurate placement of the camera before capturing the images or by a post processing stage such as image rectification, for example as disclosed in EP 1 235 439. Image rectification is a method for re-aligning the two images so that they are parallel in three dimensions. This involves reprojecting them onto a common parallel plane by a geometrical transformation. When the images are parallel in this manner, they are considered to be rectilinear. If the images are not rectilinear, then the stereo image will be uncomfortable to view due to vertical disparities, perspective effects, excessive depth, etc, that the HVS cannot reconcile and the 3D effect breaks down.
Image rectification methods usually use powerful computer vision algorithms. However if the initial pair of images are badly aligned, such algorithms will be very slow or may completely fail. If a large amount of rectification is required, then the image quality may suffer when the two images are reprojected. Furthermore, the image overlap between the rectified images may be reduced and less usable image area will be available for the 3D image. Conversely, if the initial image alignment is good, the rectification can be processed much faster and undesirable side effects from image reprojection will be reduced. Therefore, if the two initial images are taken from well aligned positions, then a more comfortable 3D image may be created and a minimal amount of rectification will be required.
2001-230955 discloses a technique which is used in the Pentax Optio 230 digital camera and which provides a user-guided two shot stereo photography mode. In this method, the user takes the first image and is advised to “move the camera to the right”. A transparent vertical or horizontal strip of the first image is superimposed on the live image. The user is meant to align the second image to the first image using this strip. However, when this technique is followed, the camera will be rotated relative to its orientation when the first image was taken so that perspective errors are introduced and the two images are not rectilinear, resulting in a 3D image of poor visual quality.
EP1085769 discloses a variable separation dual-head camera that may utilise a method of image rectification such as that disclosed in EP1089573 for determining the amount of separation. This system is based on a priori knowledge of the scene. There is currently no known automated system for providing a recommendation of the separation required for producing 3D images which may be comfortably viewed.
The term “image feature” as used hereinafter is defined to mean anything which appears in an image. This includes, for example, lines, patterns and shading. This term is being used with its conventional meaning in the technical field of vision research.